SUMMARY We propose an ultra-miniature as well as extremely compliant system that enables massive scaling in the number of neural recordings from the brain while providing a path towards large-scale neural recordings and truly chronic brain-machine interfaces (BMI). This will be achieved via two fundamental technology innovations: 1) 10 ? 100 ?m scale, free-floating, independent sensor nodes, or neural dust, that detect and report local extracellular electrophysiological data, and 2) a subcranial mm-scale interrogators that establish power and communication links with the neural dust. The interrogator array is placed beneath the skull and below the dura mater, to avoid strong attenuation of ultrasound by bone and is powered by an external transceiver via EM energy transfer. Building on an initial theoretical treatment and in-vitro validation regarding the feasibility of power coupling and backscatter communication at these scales within the brain, and more recent in-vivo experimental data showing high-fidelity transmission of electromyogram (EMG) and electroneurogram (ENG) signals from anesthetized rats, this work will provide the first in vivo demonstration that this type of recording modality is possible in the central nervous system. In the process, we aim to map the fundamental system design trade-offs and ultimate size, power, and bandwidth scaling limits of neural recording systems at the cortical level, built from low-power electronics coupled with ultrasonic power delivery and backscatter communication. The use of distributed, ultrasonic backscattering systems to record high frequency (~kHz) neural activity would pave the way for both massive scaling in the number of neural recordings from the nervous system as well as truly chronic BMIs.